Measuring devices of this type are applied especially in industrial measurements and control technology, as well as in automation and process control technology for measuring process variables.
As a function of the process variables to be measured, today, a great number of different measuring devices are applied for this, such as e.g. pressure-, temperature-, flow- and/or fill level measuring devices.
These measuring devices deliver an output signal, which corresponds to the measured value of the registered process variable. This output signal is transmitted in an industrial plant to a superordinated unit, e.g. a central control unit, such as e.g. a control room or a process control system, connected to the measuring device. As a rule, the entire process control of the manufacturing- and/or treatment process running in the industrial plant occurs via the superordinated unit, where the output signals of various measuring devices are evaluated and, based on the evaluation, control signals are produced for actuators, which control the process flow.
Preferably, measuring devices are applied, which are connectable to the superordinated unit via a single line-pair, via which the energy supply of the measuring device as well as also the signal transmission between the measuring device and the superordinated unit occurs.
The signal transmission of these devices, which are frequently referred to also as 2-wire measuring devices, occurs preferably according to standards usual in the industry.
According to one of these standards, the signal transmission occurs by setting the electrical current flowing via the line-pair from the measuring device to an electrical current value representing the measured process variable, which is then evaluated by the superordinated unit and correspondingly interpreted. For this, today, the electrical current is regularly controlled to values between 4 mA and 20 mA as a function of the measured process variable. Additionally, a communication signal can be superimposed on this electrical current representing the measured process variable. Thus, the electrical current is modulated about the value predetermined by the process variable in accordance with a predetermined communication protocol. Widely used for this today is the communication protocol defined by the HART standard, in the case of which there is superimposed on the electrical current lying between 4 mA and 20 mA a high-frequency oscillation of +/−0.5 mA carrying the information of the communication signal.
Forming another group are measuring devices connectable to a digital data bus. In the case of these measuring devices, the electrical current flowing through the line-pair is set to a predetermined electrical current value independently of the measured process variable, and there is superimposed on this electrical current the communication signal in the form a high-frequency oscillation. Known standards for this are the PROFIBUS, FOUNDATION FIELDBUS and CAN-BUS standards.
Measuring devices of this type usually have an input circuit with a line-pair composed of a supply line and a return line also connectable to the superordinated unit. During operation, energy supply of the measuring device and signal transmission, especially of an output signal representing the process variable, occurs between measuring device and superordinated unit on this line pair.
Connected to the input circuit is a measuring sensor supplied with energy via the input circuit and serving for determining the process variable and for producing a measurement signal representing the process variable.
The input circuit includes, for example, a series regulator installed in the supply line for controlling an electrical current flowing via the line-pair, and a shunt regulator connected after the series regulator and installed in a parallel branch connecting the supply line with the return line. The measuring sensor is connected to the input circuit behind the parallel branch in parallel with the shunt regulator. The shunt regulator is in the simplest case a Zener diode, via which the voltage applied to the input of the measuring sensor is predetermined.
Available to the measuring device via the line-pair is, as a rule, only a very limited amount of energy, as determined by the applied input voltage and the electrical current set by the series regulator.
Accordingly, today, especially in connection with measuring sensors with high energy requirement, such as e.g. measuring transducers of fill-level measuring devices working with microwaves or with ultrasound, methods are applied for efficient use of the available energy.
To this end, German Patent DE 10 2006 046 243 A1 discloses a method, in the case of which the measuring sensor is operated as needed, and, in times when it is not required, it is switched off or placed in a stand-by mode.
In international published application, WO 02/07124 A1, a measuring device for measuring a process variable is described, in the case of which the measurement activities of the measuring transducer are matched to the power available via the line-pair.
The measuring device includes a line-pair composed of a supply line and a return line connectable to a superordinated unit. During operation, energy supply of the measuring device and signal transmission, especially of a measurement signal, between measuring device and superordinated unit occurs via the line-pair, and an input voltage is applied to the measuring device. A measuring device variant is described, which has an input circuit with an electrical current stage installed in the supply line for controlling the electrical current flowing via the line-pair, as well as having a circuit connected after the electrical current stage and lying in a parallel branch connecting the supply line with the return line. This circuit serves to take exactly sufficient electrical current that the voltage drop across the electrical current stage is as small as possible for lessening the power loss. Correspondingly, the voltage drop across the electrical current stage is reduced to a minimum value required for operating the electrical current stage.
A further example of a measuring device is described in international published application WO 00/75904 A1. In this case, the energy supply and the output of the measured process variable occur via a single line-pair. Also in such case, the measuring device includes an input circuit, via which the thereto connected measuring sensor is supplied with energy. The input circuit includes an electrical current controller installed in the incoming supply line, which controls the electrical current flowing via the line-pair to a value representing the process variable. Behind the electrical current controller, a switching power supply is provided in the line-pair, via which the measuring transducer is fed. Also, in the case of this measuring device, the electrical current draw of the measuring sensor is controlled in such a manner for lessening the power loss that the voltage drop across the electrical current controller is as small as possible. In the case of switching power supplies with an input-side capacitance, abrupt changes of the electrical current value to be controlled could, due to the energy stored in the capacitor, lead to the fact that the output of the electrical current controller lies at a higher potential than its input. In order to prevent this, there is placed before the electrical current controller between the two lines of the line pair a further electrical current controller, which is used exclusively when required for preventing this problem.
The procedure described in the state of the art of holding the voltage drop across the input-side series regulator as small as possible offers the advantage that basically more energy is available for the measuring transducer.
It is, however, also, depending on application, connected with considerable disadvantages. One problem is that these measuring devices are more disturbance sensitive due to the small voltage drop across the series regulator. Thus, e.g. voltage fluctuations of the input voltage, such as can arise e.g. from external disturbances, from digital communication signals superimposed on a bus, or from fluctuations of the energy consumption of other bus participants connected to the same bus, can be cancelled less adequately by the series regulator, the lower the voltage drop across such is.
A further problem is that the input impedance of the measuring device is determined decisively by the voltage drop across the input-side series regulator.
A low input impedance leads, however, in the case of a connection of the measuring device to a bus, to an increased loading of the bus. If there are a number of measuring devices connected parallel to one another on a bus, then the maximum number of measuring devices connectable to the same bus is decisively limited by their input impedances. The lower the input impedances of the measuring devices, the smaller is the maximum number of connectable measuring devices parallel to one another on the same bus.
Moreover, low input impedances lead to an increased attenuation of communication signals superimposed on the electrical current flowing via the line-pair to be received by the measuring device.